Slope wave generator



Nov- 3, 1936. P. T. FARNSWORTH ET AL 2,059,219

SLOPE WAVE (1 {KNEE ATOR Filed May 5 1930 2 Sheets--Sheet l llllil IIIIIH r 5 m W M W ww m m n R mwfl KW w WK? i XX 4 a u b a 5 X [A Q h La A 0 X A C D E Nov. 3, 1936. P. T. FARNSWORTH ET AL 2,059,219

SLOPE WAVE GENERATOR Filed May 5, 1950 2 Sheets-Sheet 2 INVENTORS PH/LO 7: FARNSWORTH. 8 HARRY R. LUBCKE.

TTORNE YS Patented Nov. 3, 1936 TES UNITE PATENT QFFE 3:,

SLOPE WAVE GENERATOR of California Application May 5, 1930, Serial No. 449,985

Our invention relates to generators for supplying electric currents of sloped or saw-tooth wave form, and particularly to generators for supplying such currents to inductive circuits such as are necessary for the magnetic deflection of cathode ray streams for use in television or other purposes.

Among the objects of our invention are: First, to provide a generator which will supply current having a wave form comprising substantially straight lines of different slopes, i. e., saw-tooth waves, to highly inductive circuits; second, to provide such a generator employing vacuum tubes at extremely high eflicieney, so that large currents may be drawn from tubes having relatively small rated power output; third, to provide a generator which may readily be adjusted as to frequency, and which is stable in operation under the desired frequency adjustment; fourth, to provide a generator which is suitable for supplying scanning current for either transmitting or receiving television tubes; fifth, to supply a generator which may be adjusted for either picture frequency" or scanning frequency operation; and sixth, to provide a generator which oifers ready means of synchronizing the transmitting and receiving apparatus in a television system.

Our invention possesses numerous other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of our invention. It is to be understood that we do not limit ourselves to this disclosure of species of our invention, as we may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawings:

Figure 1 is a diagrammatic representation of a slope-wave generator of our invention as used to supply scanning current to a television receiver.

Figure 2 is a series of curves showing the wave forms generated in the various portions of the generator of our invention.

Figure 3 is a representation of a picture field as scanned by currents produced by the generator of our invention; and

Figure 4 is a similar representation of a picture field scanned by sine waves; and

Figure 5 is a diagrammatic representation of an alternative connection which may be used in conjunction with Figure 1, as substitute for the material of Figure 1 enclosed within the broken lines.

In general terms, our invention comprises an oscillator capable of producing a voltage wave having substantially the form desired. This voltage is applied to the grid of a vacuum tube, in the output circuit of which is a distorting network which, in its simplest form, comprises an inductance and a resistance in series. The voltage produced across the network is an extremely strong pulse followed by a gradual change in voltage. 5 This voltage is applied to control the output tube of the generator, the pulse being used to conrol the current of the output tube regardless of the inductive kick produced in the output circuit. while the gradual voltage change controls the current flowing in the output circuit of the tube during the more gradually sloped portion of the wave, which is used for the actual scanning of the television picture. By using the distorting circuit, and applying the extremely strong pulse negatively to the grid of 'the output tube, the current flowing thru this tube may be completely interrupted in spite of the rise of its plate voltage due to the inductive kick of the scanning coil, Since the high voltage occurs during the period of practically zero current, the power demands upon the output tube are relatively small, and the tube may be operated at an efliciency of approximately 80%. For this reason a tube which is ordinarily rated at 5 watts may be used safely to supply about 20 watts of output power.

A preferred embodiment of our invention is shown in detail in Figure 1. A condenser In is charged thru a. resistor I l by a battery or other power source l2. Bridged across the condenser is a glow tube l3, which provides an ionizable gas discharge path around the condenser.

When the voltage in the condenser rises to the breakdown point of the glow tube, the tube ionizes, and the condenser discharges almost instantaneously. Owing to the so-called negative characteristic of the tube the discharge breaks a considerable period before the condenser is fully discharged, and the condenser at once starts to charge again, the charge being gradual because 49 of the limitation of charging current by the resistor ll. Both the charge and the discharge of the condenser are, of course, logarithmic, but since the tube It de-ionizes and the discharge ceases long before the condenser is completely discharged, the wave form produced is substantially that shown in curve M, Figure 2A, the curve being referred to the axis X-X.

The frequency of the wave generated is dependent upon the capacity of the condenser I0, 50 the value of the resistor l I, and the voltage of the source i2, a. variation of the latter being perhaps the readiest means of adjusting the natural frequency of the oscillator.

This wave is impressed upon the grid of the in the opposite tube l5 thru the coupling condenser iii. A biasing resistor l1 and battery I8 are provided, so that the wave, as impressed upon the grid of the tube I5, is substantially the wave of the curve l4 as referred to the axis X'X'.

The tube [5 is preferably of the screen grid" type, and has an extremely high output impedance. Connected to the plate of this tube is a distorting circuit comprising an inductance 20 in series with a resistor 2|. The impedance of the distorting circuit is preferably so low in comparison to that of the tube which feeds it that w it has practically no effect in determining the plate current of the tube, which is therefore of the wave form shown in curve M of Figure 2B.

The voltage produced across the distorting circuit'by the current wave is the sum of the inductive and resistive voltage drops in this circuit. Mathematically,

The first term of the equation is directly proportional to the current, and is represented by the curve 22 of Figure "2C. The second term is proportional to the slope of the current wave, and is represented by curve 23 of the same figure.

Since, in the apparatus as it is preferably set up, the current increases from 10 to 20 times as rapidly as it decreases, the inductive kick or pulse in one direction is from 10 to 20 times as great as direction. The total voltage across the network is the sum of the voltage waves 22 and 23, and is shown in the curve 25 of Figure 2D.

The voltage across the network is applied thru the coupling condenser 26 to the grid of a low impedance tube 21. A grid resistor 28 is connected from grid to filament of this tube, but no biasing voltage is applied. The output of the tube may feed directly into an inductive circuit, such as the deflecting coils 30 of a cathode ray receiver tube 3| but is preferably fed to these coils thru a step-down transformer 33.

Whether the transformer is used or not, sudden changes of current in the output circuit will produce very large voltages. The voltage pulses produced across the distortingcircuit are applied negatively to the grid of the tube 21. Since these voltage pulses are of high magnitude, their, tendency is completely to suppress plate current in the tube 21. The circuit is arranged so that this complete interruption of current would normally take place at a voltage level indicated by the line of Figure 2D. The plate voltage of the tube,

"however, rises simultaneously with the control voltage pulse, due to the inductance in the circuit, and hence all, or nearly all of the control voltage pulse is necessary completely to interrupt the plate current.

The current is therefore suddenly interrupted by thepulse, and thereafter increases gradually in accordance with the gradually rising control voltage applied to the grid by the sloped portion of the control voltage wave, and the resulting current is very nearly of the wave form shown in curve 36 of Figure 2E. This wave is practically undistorted by the curvature of the lower portion of the characteristic curve of the tube 21, since the fallof the voltage pulse is even more rapid than its rise, and. since it may be made to drop to a value in the straight line portion of the characteristic curve practically instantaneously.

It is usually advisable to bridge the transformer 33, or the coils 30 if the transformer be not used, by a resistor 31. The function of this resistor is to prevent oscillations of high frequency in the output circuit, due to shock excitation from the voltage pulses. It has the effect of slightly rounding the angles of the saw-tooth current wave, and decreasing the slope of the return wave slightly. Since the transformer 33 and the coils 30 are made of as low resistance as possible, the voltage wave produced across transformer and coils is of the general form shown in curve 38 of Figure 2E.

The method of application of the slope-wave generator to a television receiving system is indicated in Figure 1. The radio wave, modulated by the picture current, is received upon the antenna 40 and passed thru an amplifier-detector 4| having a power output tube 42. This tube is coupled by means of the plate resistor 43, grid resistor and coupling condenser 46 to the control grid 41 of the cathode ray tube 3|.

As is the usual practice, the anode 48 is supplied with a high voltage from the battery or other source 50 and the cathode ray stream produced by the filament 5! is modulated by the control electrode and is deflected across the fluorescent screen 52 by the current in the coils 30.

As is described in the co-pending Philo T. Farnsworth application, Serial No. 449,984 filed May 5, 1930, an oscillator similar to the one just described is used to produce scanning at the transmitter, and the voltage pulses produced in the scanning coils are utilized to cut the picture current to zero during the period of the steep return slope of the scanning wave. The result of this arrangement is that picture currents may modulate the carrier approximately 30%, while the pulses corresponding to the return wave produce a 100% modulation. These pulses are applied negatively to the grid of the receiving tube, and hence interrupt the scanning beam at the receiver during the same period.

. The result is that a picture field is produced which is of the type shown in exaggerated form in Figure 3, in contra-distinction to the field produced by the ordinary sine wave scanning which is shown in Figure 4.

Returning again to Figure 1, a resistor 55 is connected between the control electrode 41 and one terminal of the condenser III of the oscillator. The resistor 55 is preferably of about four times the magnitude of the resistor II, and may be of the order of a megohm, while the resistors 43 and 45 are of the order of 2,000 ohms and 50,000 ohms respectively. The voltage applied to the control electrode 41 by the pulses is of the order of 30 volts, and this voltage, applied to the resistors 55 and II in series, produces about 5 volts across the condenser I. A voltage of this magnitude will hold the oscillation produced by the condenser and glow tube in synchronism thru a range of plus or minus ten percent in natural frequency.

On the other hand, due to the very high value of the resistor 55 as compared to resistors 43 and 45, the voltage of the oscillator has little or no effect upon the picture current.

One of the chief advantages of the oscillator here described is the high working efliciency of the tube 21. During the periods of very high voltage no current is flowing thru the tube. Maximum current flow occurs at the period of minimum plate voltage, and therefore the tube losses, which appear as heating of the plate, are a minimum. Tube efficiencies of '70 to 80% and even higher may be produced with this arrangement. The rating of a tube is dependent upon the amount of power which may be dissipated by its plate circuit, and ratings are predicated upon sine wave operations, in which the tube efliciency does not exceed 50% A 5 watt tube will therefore safely dissipate 5 watts in its plate circuit. If the tube be operated at a plate circuit efliciency of 80%, this 5 watts represents but 20% of the total power, and hence a 5 watt tube may safely be used to deliver 20 volt amperes in the circuit described.

Figures 3 and 4 show the advantages of using currents of the wave form here described for scanning a picture iield. Not only is the dis- .tribution of the field better, but slight phase displacements between transmitter and receiver have less serious consequences. Thus when a .line 56 is scanned with a sine wave 51, a slight phase lag at the receiver will cause the received image to separate into two, occupying the positions shown by the dots 58. This spoils the definition of the picture, however small the phase difference may be. With our system a slight phase difference will merely shift the image of the line 60 to a new position 6|.

We claim:

1. A slope-wave generator comprising a condenser, means for gradually charging and suddenly discharging said condenser, a vacuum tube, means for applying a voltage derived from said charge and discharge to control the plate current of said vacuum tube, a distorting network in the output of said tube, a second tube having an input circuit connected to said distorting network, and an output circuit for said second tube.

I 2. A slope-wave generator comprising a condenser, means for gradually charging and suddenly discharging said condenser, a vacuum tube,

means for applying a voltage derived from said charge and discharge to control the plate current of said vacuum tube, a network in the output of said tube comprising an inductor and a resistor in series, a second tube having a control circuit connected across said network, and an output circuit for said second tube.

3. A slope-wave generator comprising a condenser, means for gradually charging and suddenly discharging said condenser, a vacuum tube, means for applying a voltage derived from said charge and discharge to control the plate current of said vacuum tube, a distorting network in the output of said tube, a second tube having an input circuit connected to said distorting network, an inductive circuit connected in the output of said second tube, and means for preventing oscillation of said circuit due to the inductive kick induced by sudden pulses therein.

4. A slope-wave generator comprising a condenser, means for gradually charging and suddenly discharging said condenser, a vacuum tube, means for applying a voltage derived from said charge and discharge to control the plate current 'of said vacuum tube, a distorting network in the output of said, tube, a second tube having an input circuit connected to said distorting network, an inductive circuit connected in the output of said second tube, and a leak path bridging said inductive circuit to prevent free oscillations therein.

5. A slope-wave generator comprising a condenser, a resistor, a voltage source connected to charge said condenser thru said resistor, an ionizable gas discharge path connected to discharge said condenser, a vacuum tube connected for controlby the condenser voltage to pass an output current varying therewith, a distorting network forming an output circuit for said tube, a current source of susbtantially constant voltage, and means actuated by voltage in said distube, and a low impedance output tube connected for actuation by the voltage across said distorting network.

'7. A slope-wave generator comprising a high impedance vacuum tube, means for supplying a slope-wave potential to the control electrode of said tube, an output circuit for said tube having inductance and resistance and of relatively low impedance as compared to the tube, and an output tube of relatively low impedance connected for actuation by the voltage across said circuit.

8. The method of generating a current having a wave form comprising substantially straight lines of unequal slopes, which comprises generating a voltage wave of similar form, using said voltage wave to control a current to produce a similar wave form, distorting the voltage corre sponding to said current wave to produce a wave comprising intense pulses succeeded by gradual voltage changes, and controlling a unidirectional current flow by said last mentioned wave to produce the current wave desired.

9. The method of generating a current having a wave form comprising substantially straight lines of unequal slopes, which comprises generating a voltage wave of similar form, using said voltage wave to control a current to produce a similar wave form, distorting the voltage corresponding to said current wave to produce a wa' comprising intense negative pulses succeeded by gradual Voltage rises, and controlling a unidirectional current fiow by said last mentioned wave to produce the current wave desired.

10. A slope-wave generator comprising a condenser, meansfor charging and discharging said condenser at different rates, a vacuum tube, means for applying a voltage derived from said charge and discharge to control the plate current of said vacuum tube, a distorting network in the output of said tube, a second tube having an input circuit connected to said distorting network, and an output circuit for said second tube.

11. A slope-wave generator comprising a condenser, means for charging and discharging said condenser at different rates, a vacuum tube, means for applying a voltage derived from said charge and discharge to control the plate current of said vacuum tube, a network in the output of said tube comprising an inductor and a resistor in series, a second tube having a control circuit connected across said network, and an output circuit for said second tube.

12. A slope-wave generator comprising a condenser, a resistor, a voltage source connected to charge said condenser through said resistor, an ionizable gas discharge path connected to discharge said condenser, a vacuum tube connected for control by the condenser voltage to pass an output current varying therewith, a distorting network forming an output circuit for said tube, said network comprising an inductor and a resistor in series, a current source of substantially constant voltage, and means actuated by voltage in said distorting network for controlling the current flow from said source.

' 13. A slope-wave generator comprising a condenser, means for charging said condenser slowly, means for discharging said condenser suddenly, a vacuum tube, means for applying a voltage derived from said charge and discharge to control the plate current of said vacuum tube, a distorting network in the output of said tube, a second tube having an input circuit connected to said distorting network, and an output circuit for said second tube.

14. The method of converting a slope-wave voltage into a slope-wave current in a succession of vacuum tubes feeding into a reactive output circuit which comprises the steps of generating a voltage slope wave, feeding said wave to one of said tubes, distorting the output of said last men-v tioned tube, and utilizing said distorted output to control the output of the tube feeding said reactive circuit to produce said slope-wave current.

15. In the art of developing from a given voltage wave a similar current wave in a given electrical circuit, the method which comprises utilizing the given voltage wave to force a similar current wave through an electrical circuit whose frequency-impedance characteristic is similar to that of the first-mentioned circuit, amplifying the back E. M. F. voltage across the second-mentioned circuit, and applying such amplified voltage to the first-mentioned circuit.

16. In the art of television wherein deflecting eiectro-magnetic means operates to deflect a cathode ray, the method of developing in said means a saw-tooth current wave; said method comprising developing a saw-tooth voltage wave, utilizing the voltage wave to force a saw-tooth current wave through an electrical circuit whose frequency-impedance characteristic is similar to that of said means, amplifying the back E. M. F. voltage across said circuit, and applying such amplified voltage to said means to develop therein the required saw-tooth current wave.

17. In a system for television, a cathode ray tube, electromagnetic means for deflecting the ray, a thermionic device having a, grid, means for applying to said grid a. sawtooth voltage wave, an electrical circuit connected in the plate circuit of said device, and an amplifier tube having a grid, said electromagnetic means connected in the plate circuit of said tube, said electrical circuit having a frequency-impedance characteristic substantially similar to such characteristic of said electromagnetic means.

PHILO T. FARNSWORTH. HARRY R. LUBCKE. 

